Human
Humans commonly refers to the species Homo sapiens (Latin: "wise man" or "knowing man"), the only extant member of the Homo genus of bipedal primates in Hominidae, the great ape family. However, in some cases the term is used to refer to any member of the genus Homo. Humans have a highly developed brain, capable of abstract reasoning, language, introspection, and problem solving. This mental capability, combined with an erect body carriage that frees the hands for manipulating objects, has allowed humans to make far greater use of tools than any other species. Mitochondrial DNA and fossil evidence indicates that modern humans originated in Africa about 200,000 years ago. Humans are a cosmopolitan species; widespread in every continent except Antarctica, with a total population of 6.8 billion as of November 2009. Like most higher primates, humans are social by nature. However, humans are uniquely adept at utilizing systems of communication for self-expression, the exchange of ideas, and organization. Humans create complex social structures composed of many cooperating and competing groups, from families to nations. Social interactions between humans have established an extremely wide variety of values, social norms, and rituals, which together form the basis of human society. Humans are noted for their desire to understand and influence their environment, seeking to explain and manipulate natural phenomena through science, philosophy, mythology and religion. This natural curiosity has led to the development of advanced tools and skills, which are passed down culturally; humans are the only species known to build fires, cook their food, clothe themselves, and use numerous other technologies. History The scientific study of human evolution encompasses the development of the genus Homo, but usually involves studying other hominids and hominines as well, such as Australopithecus. "Modern humans" are defined as the Homo sapiens species, of which the only extant subspecies is known as Homo sapiens sapiens. Homo sapiens idaltu (roughly translated as "elder wise human"), the other known subspecies, is now extinct. Homo neanderthalensis, which became extinct 30,000 years ago, has sometimes been classified as a subspecies, "Homo sapiens neanderthalensis", but genetic studies now suggest a divergence of the Neanderthal species from Homo sapiens about 500,000 years ago. Similarly, the few specimens of Homo rhodesiensis have also occasionally been classified as a subspecies, but this is not widely accepted. Anatomically modern humans first appear in the fossil record in Africa about 195,000 years ago, and studies of molecular biology give evidence that the approximate time of divergence from the common ancestor of all modern human populations was 200,000 years ago. The broad study of African genetic diversity headed by Dr. Sarah Tishkoff found the San people to express the greatest genetic diversity among the 113 distinct populations sampled, making them one of 14 "ancestral population clusters". The research also located the origin of modern human migration in south-western Africa, near the coastal border of Namibia and Angola. The closest living relatives of humans are gorillas and chimpanzees, but humans did not evolve from these apes: instead these apes share a common ancestor with modern humans.18 Humans are probably most closely related to two chimpanzee species: Common Chimpanzee and Bonobo. Full genome sequencing has resulted in the conclusion that "after 6.5 million years of separate evolution, the differences between chimpanzee and human are ten times greater than those between two unrelated people and ten times less than those between rats and mice". Suggested concurrence between human and chimpanzee DNA sequences range between 95% and 99%. It has been estimated that the human lineage diverged from that of chimpanzees about five million years ago, and from that of gorillas about eight million years ago. However, a hominid skull discovered in Chad in 2001, classified as Sahelanthropus tchadensis, is approximately seven million years old, which may indicate an earlier divergence. Human evolution is characterized by a number of important morphological, developmental, physiological and behavioural changes, which have taken place since the split between the last common ancestor of humans and chimpanzees. The first major morphological change was the evolution of a bipedal locomotor adaptation from an arboreal or semi-arboreal one, with all its attendant adaptations, such as a valgus knee, low intermembral index (long legs relative to the arms), and reduced upper-body strength. Later, ancestral humans developed a much larger brain – typically 1,400 cm³ in modern humans, over twice the size of that of a chimpanzee or gorilla. The pattern of human postnatal brain growth differs from that of other apes (heterochrony), and allows for extended periods of social learning and language acquisition in juvenile humans. Physical anthropologists argue that the differences between the structure of human brains and those of other apes are even more significant than their differences in size. Other significant morphological changes included: the evolution of a power and precision grip; a reduced masticatory system; a reduction of the canine tooth; and the descent of the larynx and hyoid bone, making speech possible. An important physiological change in humans was the evolution of hidden oestrus, or concealed ovulation, which may have coincided with the evolution of important behavioural changes, such as pair bonding. Another significant behavioural change was the development of material culture, with human-made objects becoming increasingly common and diversified over time. The relationship between all these changes is the subject of ongoing debate. The forces of natural selection have continued to operate on human populations, with evidence that certain regions of the genome display directional selection in the past 15,000 years. Anatomically modern humans evolved from archaic Homo sapiens in Africa in the Middle Paleolithic, about 200,000 years ago. By the beginning of the Upper Paleolithic 50,000 BP (Before Present), full behavioral modernity, including language, music and other cultural universals had developed. The broad study of African genetic diversity headed by Dr. Sarah Tishkoff found the San people to express the greatest genetic diversity among the 113 distinct populations sampled, making them one of 14 "ancestral population clusters". The research also located the origin of modern human migration in south-western Africa, near the coastal border of Namibia and Angola. The out of Africa migration is estimated to have occurred about 70,000 years BP. Modern humans subsequently spread to all continents, replacing earlier hominids: they inhabited Eurasia and Oceania by 40,000 BP, and the Americas at least 14,500 years BP. They displaced Homo neanderthalensis and other species descended from Homo erectus (which had inhabited Eurasia as early as 2 million years ago) through more successful reproduction and competition for resources. Evidence from archaeogenetics accumulating since the 1990s has lent strong support to the "out-of-Africa" scenario, and has marginalized the competing multiregional hypothesis, which proposed that modern humans evolved, at least in part, from independent hominid populations. Geneticists Lynn Jorde and Henry Harpending of the University of Utah propose that the variation in human DNA is minute compared to that of other species. They also propose that during the Late Pleistocene, the human population was reduced to a small number of breeding pairs – no more than 10,000, and possibly as few as 1,000 – resulting in a very small residual gene pool. Various reasons for this hypothetical bottleneck have been postulated, one being the Toba catastrophe theory. The rise of agriculture, and domestication of animals, led to stable human settlements. The path followed by humans in the course of history Until c. 10,000 years ago, most humans lived as hunter-gatherers. They generally lived in small nomadic groups known as band societies. The advent of agriculture prompted the Neolithic Revolution, when access to food surplus led to the formation of permanent human settlements, the domestication of animals and the use of metal tools. Agriculture encouraged trade and cooperation, and led to complex society. Because of the significance of this date for human society, it is the epoch of the Holocene calendar or Human Era. About 6,000 years ago, the first proto-states developed in Mesopotamia, Egypt's Nile Valley and the Indus Valleys. Military forces were formed for protection, and government bureaucracies for administration. States cooperated and competed for resources, in some cases waging wars. Around 2,000–3,000 years ago, some states, such as Persia, India, China, Rome, and Greece, developed through conquest into the first expansive empires. Influential religions, such as Judaism, originating in the Middle East, and Hinduism, a religious tradition that originated in South Asia, also rose to prominence at this time. The late Middle Ages saw the rise of revolutionary ideas and technologies. In China, an advanced and urbanized society promoted innovations and sciences, such as printing and seed drilling. In India, major advancements were made in mathematics, philosophy, religion and metallurgy. The Islamic Golden Age saw major scientific advancements in Muslim empires. In Europe, the rediscovery of classical learning and inventions such as the printing press led to the Renaissance in the 15th and 16th centuries. Over the next 500 years, exploration and colonialism brought much of the Americas, Asia, and Africa under European control, leading to later struggles for independence. The Scientific Revolution in the 17th century and the Industrial Revolution in the 18th–19th centuries promoted major innovations in transport, such as the railway and automobile; energy development, such as coal and electricity; and government, such as representative democracy and Communism. With the advent of the Information Age at the end of the 20th century, modern humans live in a world that has become increasingly globalized and interconnected. As of 2008, over 1.4 billion humans are connected to each other via the Internet, and 3.3 billion by mobile phone subscriptions. Although interconnection between humans has encouraged the growth of science, art, discussion, and technology, it has also led to culture clashes and the development and use of weapons of mass destruction. Human civilization has led to environmental destruction and pollution, producing an ongoing mass extinction of other forms of life called the holocene extinction event, that may be further accelerated by global warming in the future. Biology Human body types vary substantially. Although body size is largely determined by genes, it is also significantly influenced by environmental factors such as diet and exercise. The average height of an adult human is about 1.5 to 1.8 m (5 to 6 feet) tall, although this varies significantly from place to place. The average mass of an adult human is 54–64 kg (120–140 lbs) for females and 76–83 kg (168–183 lbs) for males. Weight can also vary greatly (e.g. obesity, anorexia). Unlike most other primates, humans are capable of fully bipedal locomotion, thus leaving their arms available for manipulating objects using their hands, aided especially by opposable thumbs. Although humans appear hairless compared to other primates, with notable hair growth occurring chiefly on the top of the head, underarms and pubic area, the average human has more hair follicles on his or her body than the average chimpanzee. The main distinction is that human hairs are shorter, finer, and less heavily pigmented than the average chimpanzee's, thus making them harder to see. The hue of human skin and hair is determined by the presence of pigments called melanins. Human skin hues can range from very dark brown to very pale pink. Human hair ranges from white to brown to red to most commonly black. This depends on the amount of melanin (an effective sun blocking pigment) in the skin and hair, with hair melanin concentrations in hair fading with increased age, leading to grey or even white hair. Most researchers believe that skin darkening was an adaptation that evolved as a protection against ultraviolet solar radiation. However, more recently it has been argued that particular skin colors are an adaptation to balance folate, which is destroyed by ultraviolet radiation, and vitamin D, which requires sunlight to form. The skin pigmentation of contemporary humans is geographically stratified, and in general correlates with the level of ultraviolet radiation. Human skin also has a capacity to darken (sun tanning) in response to exposure to ultraviolet radiation. Humans tend to be physically weaker than other similarly sized primates, with young, conditioned male humans having been shown to be unable to match the strength of female orangutans which are at least three times stronger.52Constituents of the human body In a person weighing 60 kg Constituent Weight53 Percentage of atoms53 Oxygen 38.8 kg 25.5% Carbon 10.9 kg 9.5% Hydrogen 6.0 kg 63.0% Nitrogen 1.9 kg 1.4% Other 2.4 kg 0.6% Humans have proportionately shorter palates and much smaller teeth than other primates. They are the only primates to have short, relatively flush canine teeth. Humans have characteristically crowded teeth, with gaps from lost teeth usually closing up quickly in young specimens. Humans are gradually losing their wisdom teeth, with some individuals having them congenitally absent. Human physiology is the science of the mechanical, physical, and biochemical functions of humans in good health, their organs, and the cells of which they are composed. The principal level of focus of physiology is at the level of organs and systems. Most aspects of human physiology are closely homologous to corresponding aspects of animal physiology, and animal experimentation has provided much of the foundation of physiological knowledge. Anatomy and physiology are closely related fields of study: anatomy, the study of form, and physiology, the study of function, are intrinsically tied and are studied in tandem as part of a medical curriculum. Humans are a eukaryotic species. Each diploid cell has two sets of 23 chromosomes, each set received from one parent. There are 22 pairs of autosomes and one pair of sex chromosomes. By present estimates, humans have approximately 20,000–25,000 genes. Like other mammals, humans have an XY sex-determination system, so that females have the sex chromosomes XX and males have XY. The X chromosome carries many genes not on the Y chromosome, which means that recessive diseases associated with X-linked genes, such as haemophilia, affect men more often than women. The human life cycle is similar to that of other placental mammals. The zygote divides inside the female's uterus to become an embryo, which over a period of thirty-eight weeks (9 months) of gestation becomes a human fetus. After this span of time, the fully grown fetus is birthed from the woman's body and breathes independently as an infant for the first time. At this point, most modern cultures recognize the baby as a person entitled to the full protection of the law, though some jurisdictions extend various levels of personhood earlier to human fetuses while they remain in the uterus. Compared with other species, human childbirth is dangerous. Painful labors lasting twenty-four hours or more are not uncommon and sometimes leads to the death of the mother, or the child. This is because of both the relatively large fetal head circumference (for housing the brain) and the mother's relatively narrow pelvis (a trait required for successful bipedalism, by way of natural selection). The chances of a successful labor increased significantly during the 20th century in wealthier countries with the advent of new medical technologies. In contrast, pregnancy and natural childbirth remain hazardous ordeals in developing regions of the world, with maternal death rates approximately 100 times more common than in developed countries. In developed countries, infants are typically 3–4 kg (6–9 pounds) in weight and 50–60 cm (20–24 inches) in height at birth. However, low birth weight is common in developing countries, and contributes to the high levels of infant mortality in these regions. Helpless at birth, humans continue to grow for some years, typically reaching sexual maturity at 12 to 15 years of age. Females continue to develop physically until around the age of 18, whereas male development continues until around age 21. The human life span can be split into a number of stages: infancy, childhood, adolescence, young adulthood, adulthood and old age. The lengths of these stages, however, have varied across cultures and time periods. Compared to other primates, humans experience an unusually rapid growth spurt during adolescence, where the body grows 25% in size. Chimpanzees, for example, grow only 14%, with no pronounced spurt. The presence of the growth spurt is probably necessary to keep children physically small until they are psychologically mature. Humans are one of the few species in which females undergo menopause. It has been proposed that menopause increases a woman's overall reproductive success by allowing her to invest more time and resources in her existing offspring and/or their children (the grandmother hypothesis), rather than by continuing to bear children into old age. There are significant differences in life expectancy around the world. The developed world is generally aging, with the median age around 40 years (highest in Monaco at 45.1 years). In the developing world the median age is between 15 and 20 years. Life expectancy at birth in Hong Kong is 84.8 years for a female and 78.9 for a male, while in Swaziland, primarily because of AIDS, it is 31.3 years for both sexes. While one in five Europeans is 60 years of age or older, only one in twenty Africans is 60 years of age or older. The number of centenarians (humans of age 100 years or older) in the world was estimated by the United Nations at 210,000 in 2002. At least one person, Jeanne Calment, is known to have reached the age of 122 years; higher ages have been claimed but they are not well substantiated. Worldwide, there are 81 men aged 60 or older for every 100 women of that age group, and among the oldest, there are 53 men for every 100 women. Category:Humans Category:Species